The present invention relates to above-ground storage tanks and to the prevention, detection and/or repair of leaks or avenues for leaks from the bottoms of such tanks.
One approach to preventing leaks taken in the construction of new above-ground storage tanks involves placing a plastic liner inside a supporting ring wall-type concrete foundation and underneath the tank bottom (with the tank bottom forming, of course, the primary barrier to leaks of the tank contents to groundwater and the tank's immediate environment). On many existing storage tanks, however, the tank bottom is effectively the only barrier between the tank's contents and the environment.
As to these existing above-ground tanks, there are a limited number of options available for repairing leaks which develop in the tank, or in the absence of a known leak for adding a second barrier between a tank's contents and the environment to prevent leaks to the environment. One option is to replace and reconstruct the tank entirely. A second option is to lift the tank according to U.S. Pat. Nos. 4,807,851 and 4,930,750 and place a plastic liner inside the foundation and under the tank's bottom as if it were a new construction. A third option, and the option most frequently elected because of the expense, time and difficulty associated with the other two options, is to install a second bottom above the original bottom.
Generally this second bottom is welded to the walls of the tank above the original bottom, and on top of a bed of sand or other granular supporting material. For the tank's contents to reach the environment, the materials in question must leak through the second bottom or through the welds between the second bottom and the tank wall, through the sand bed, and through the original bottom.
In trying to prevent or minimize leaks to the environment, and since the sand bed defines paths for movement of the tank's contents therethrough, the sand bed of a given retrofitted tank conventionally is monitored for the presence therein of the tank's contents via nozzles or ports placed in the side of the tank and in fluid communication with the sand bed. When a leak through the second bottom does occur, however, the sand bed must be regenerated by removing and replacing the contaminated sand. This is a labor intensive, costly exercise, and in addition generates a great deal of contaminated sand for incineration or landfilling.